As disclosed in Japanese Patent Application Laid-Open Publication No. 2004-92634, engine driven vehicles such as ATVs (All Terrain Vehicles, so-called buggies), tractors, or recreation vehicles that are vehicles intended mainly for driving on rough ground have been incorporating a power generator set driven by an engine for driving a vehicle to supply commercial AC electric power of AC 100 V or AC 200 V (50 Hz or 60 Hz), or the like to an external electrical load such as an electric tool or a home appliance while the vehicle is stopped, in order to allow the external electrical load to be used outdoors.
As known, a vehicle-mounted power generator set is comprised of an AC generator driven by an engine and an electric power conversion portion that converts an output voltage of the AC generator into an AC voltage having a fixed frequency (generally, commercial frequency). An electric power conversion portion comprised of a converter that converts an output of the AC generator into a DC output and an inverter that converts the DC output from the converter into an AC output having commercial frequency, or an electric power conversion portion comprised of a cyclone converter that directly converts an output of the AC generator into an AC output having commercial frequency is used.
In such a power generator set, even if a rotational speed of a generator is changed, an AC output having a fixed frequency can be obtained, and thus the rotational speed of the generator can be freely changed. When a load is light, the rotational speed of the engine can be reduced to improve specific fuel consumption (efficiency), and when the load is heavy, the rotational speed of the engine can be increased to increase the amount of electric power generated. Thus, a high output can be obtained without having a very large generator. Also, the amount of electric power generated can be controlled according to the rotational speed of the engine, and thus a magnetic AC generator that cannot control a magnetic field can be used as a generator for supplying electric power to an external electrical load. The magnetic AC generator is widely used as a generator for supplying electric power to various electrical components (an ignition device of an engine, a fuel injection device, a controller for controlling the ignition device and the fuel injection device, or the like) provided in a vehicle body. A vehicle-mounted power generator set for supplying electric power to an external load can be easily comprised by providing an armature winding for supplying electric power to the external load to the magnetic AC generator.
In the vehicle-mounted power generator set, a control mode of the engine is switched from a vehicle driving mode to a power generation mode by a mode selection switch when the vehicle is stopped and a gear position of a power transmission device is in a position for cutting power transmission (a neutral position or a parking position). When the control mode of the engine is switched to the power generation mode, the controller increases the rotational speed of the engine and controls the rotational speed of the engine at a rotational speed suitable for the external electrical load.
The vehicle-mounted power generator set that supplies electric power to the external electrical load while the vehicle is stopped is maintained in a no-load state while the vehicle is driving. Thus, if the magnetic AC generator is used as the generator, a high no-load voltage is induced in an armature coil in the generator when the rotational speed of the engine increases while the vehicle is driving. The no-load induced voltage is applied to the electric power conversion portion, and may damage circuit elements that constitute the electric power conversion portion.
Thus, as disclosed in Japanese Patent Application Laid-Open Publication No. 2004-104854, as a vehicle-mounted power generator set, an inverter controlled power generator set has been proposed including means for preventing a magnetic AC generator from generating a high no-load induced voltage when a rotational speed of an engine increases while a vehicle is driving. The power generator set disclosed in Japanese Patent Application Laid-Open Publication No. 2004-104854 includes a load resistance connected across output terminals of the magnetic AC generator via switch means, and control means for controlling to maintain the switch means in an OFF state when an output voltage of the generator is less than a set value, and switch the switch means to an ON state when the output voltage of the generator exceeds the set value.
Comprised as described above, when the rotational speed of the engine increases while the vehicle is driving and the output voltage of the generator increases, the load resistance can be connected across the output terminals of the generator to pass a current from the generator through the load resistance, thereby reducing the output voltage of the generator and preventing an overvoltage from being applied to the electric power conversion portion.
For the power generator set in Japanese Patent Application Laid-Open Publication No. 2004-104854, when the rotational speed of the engine increases while the vehicle is driving, the current passes from the generator through the load resistance and electric power is wasted in the load resistance. This may increase the load of the engine when the engine rotates at a high speed, thereby reducing an engine output provided to drive wheels of the vehicle or reducing fuel economy.